The present invention relates to a phase-locked loop circuit and a data reproducing apparatus to reproduce synchronizing clock pulses in a digital audio disc player for use with an optical compact disc.
A digital recording and reproduction technique in the field of audio equipment making use of pulse code modulation (hereinafter PCM) has been employed more frequently to achieve the most faithful sound reproduction possible. This technique is called a digital audio system because it has been found in principle that digital audio characteristics do not depend on the recording medium used and are much superior to the conventional analog recording and reproduction. The digital audio system which uses a disc as a recording medium is known as a digital audio disc system.
Various recording and reproducing means including optical, electrostatic and mechanical means have been proposed for digital audio systems. For example, the optical system uses a transparent disc of resin having a diameter of 12 cm and a thickness of 1.2 mm. The disc is covered with a thin metal coating in which pits corresponding to audio signal are formed to exhibit different reflectiveness at different disc locations. The audio signal in an optical system is subjected to a given eight to fourteen modulation (hereinafter EFM), interleaved and converted into a digital signal form by a PCM technique. When the digital signal is reproduced from the disc, the disc is rotated at a constant linear velocity but at a variable rotational velocity of about 200-500 rpm. An optical pickup device incorporating a semiconductor laser and a photoelectric conversion element is operated such that the laser beam is directed from the inner side of the disc toward the outer side to effect a linear tracking.
To reconvert the data in digital form obtained from the optical pickup device back to the original analog audio signal, the EFM signal is obtained by shaping the data reproduced by the optical pickup. A clock signal used for synchronizing signals in various sections of the reproduction system is reproduced from the FM signal through the use of a phase-locked loop (hereinafter PLL). In this case, a phase comparator of the PLL compares the phase of the clock signal from a VCO of the PLL with the phase of the EFM signal to phase lock the two signals. Since the EFM signal is subjected to EFM modulation, the interval at which the polarity inversion takes place varies from 3 bits to 11 bits, assuming that one period of the clock signal is one bit.
FIG. 1 shows a conventional data reproducing system for obtaining a synchronized EFM signal and a clock signal from an input pulse signal obtained from a reproduction system. This reproducing system is explained in greater detail below. The conventional data reproduction system fails to achieve a precise EFM signal and an accurately phase-locked clock signal because the input signal of the circuit, that is, the digital data reproduced from the optical pickup, has a very irregular period from 3 bits to 11 bits. The conventional data means for producing the EFM signal and the clock signal tend to introduce considerable error in the output data.